Hundreds of millions of birds collide with windows annually, with estimates suggesting up to a billion perish in North America. Glass presents a significant challenge for birds, who navigate their world through different visual cues than humans. Understanding why these collisions occur is the first step toward effective solutions.
How Birds Perceive Glass
Birds generally do not perceive glass as a solid barrier. Their difficulty stems from two properties: transparency and reflectivity. Birds may see through clear glass to habitat or sky, believing it an unobstructed flight path. Conversely, glass often acts like a mirror, reflecting images of trees, open sky, or other natural elements, tricking birds into thinking they are flying into continuous habitat or open space.
Avian vision, while sharp and capable of seeing a broader spectrum of colors including ultraviolet light for some species, has limitations regarding depth perception on flat surfaces. Birds typically have eyes positioned on the sides of their heads, granting them a wide field of view to detect predators. This wide view compromises their binocular vision and ability to accurately judge distances to flat objects directly in front of them. Glass does not exist in their natural environment, so birds have not evolved to recognize it as a physical obstacle. They rely on cues like textures, shadows, and movement, which clear glass lacks.
Reasons for Collisions
Bird collisions are caused by their misinterpretation of glass as open space or continuous environment. Birds may attempt to fly towards what appears to be open sky or a continuous forest due to reflections. Transparent windows can create a “tunnel effect,” where birds see through a building to another window or outdoor area, mistakenly perceiving a clear flight path. This illusion can lead birds to fly at full speed directly into the surface.
Sudden scares, such as being chased by a predator or chasing prey, can lead birds to fly erratically into windows. During high-stress moments, their instinct to escape or capture overrides their ability to discern barriers.
Migration is a factor, particularly for species that travel at night. Brightly lit buildings in urban landscapes can disorient migrating birds, drawing them off course and making them vulnerable to collisions with illuminated or reflective windows.
Bird feeder placement can contribute to collisions. Feeders located between three and thirty feet from a window allow birds to build momentum for serious injury if startled. Interior plants visible through windows can attract birds, leading them to fly into the glass. Some territorial birds may repeatedly attack their own reflection, mistaking it for a rival.
Preventing Window Strikes
Making windows visible to birds is the most effective way to prevent collisions. Applying visual deterrents to the outside surface of windows is effective, as reflections can obscure markers placed on the inside. Patterns of decals, tapes, or dots should be spaced no more than two inches apart horizontally and two inches apart vertically, to signal to birds the space is too small to fly through. Products like Feather Friendly tape or Solyx films offer pre-patterned options.
Installing external screens or netting over windows provides a physical barrier and breaks up reflections, making the glass visible. Window treatments such as external sunshades, awnings, or angled windows can reduce reflectivity. Strategic landscaping involves avoiding trees or shrubs that reflect prominently in windows, or positioning them to avoid misleading reflections.
Feeders should be placed either very close to windows (within three feet) so birds cannot build speed for a fatal impact, or far away (beyond thirty feet) to allow ample maneuver space. Reducing artificial light at night, especially during migration seasons, prevents disorienting birds. For new construction, bird-friendly materials like fritted glass or UV-reflective coatings visible to birds offer solutions.